With the advancement of manufacturing ability for electronic products, an integrated circuit (IC) has been developed as the most important component of the electronic products for a digitalized society. Digital electric appliances, personal computers, and center data exchange systems for complicated processing of signals and data are all comprised of integrated circuits. Generally, for protection, an integrated circuit is enclosed in a package structure for the integrated circuit contains electric circuits that are often susceptible to physical damages. The package structure also provides electrical connection between the electrical circuit and external circuits through which power and signals can be supplied from or transmitted to the external circuits. In addition, the package structure also provides a heat dissipation interface for the integrated circuit. An integrated circuit often contains a great number of transistors and logic gates, as well as other electronic devices or components, and is operated at an extremely high speed or clock frequency, which causes a significant heat emission. Decreasing thermal resistance of the package for increasing heat dissipation efficiency is an important issue for the IC packages. The removal of heat emission from the IC, if not properly addressed, will adversely affect reliability of the IC and eventually shortening the life span of the IC.
A variety of integrated circuit packages are available, including Thin Small Outline Package (TSOP) and Quad Flat Package (QFP), which comprise a metal leadframe that support the package and pins formed on two sides or four sides of the package to connect to a circuit board on which the IC is disposed. On the other hand, a Ball Grid Array (BGA) package is connected with a circuit board by solder balls, rather than pins. Different heat dissipation solutions are required for different types of package structures. Moreover, many solutions have been developed in order to increase the heat dissipation efficiency for various types of package structures, such as changing the structure design, utilizing a high thermal conductivity material and the likes, are all used for removing heat from the package structures. However, feasible changes and modification of the structure and the material are limited, and the heat dissipation efficiency is getting more and more insufficient with the rapid increase of the number of electronic devices or components comprised in an IC. One of the most commonly seen solutions that are developed to address the above discussed problems is to dispose a heat slug inside the IC package structure for increasing the heat dissipation efficiency.
Referring to FIGS. 1 and 2, wherein FIG. 1 is a schematic side elevational view of a QFP structure, designated at 10, containing a conventional heat slug 18, and FIG. 2 is a top view of the conventional heat slug 18, as shown in FIG. 1, the QFP structure 10 comprises a die 12, a leadframe 14, a mold compound 16 and the heat slug 18. The leadframe 14 has a plurality of leadframe finger 14a and a die attach pad 14b. The die 12 is disposed on the die attach pad 14b of the leadframe 14 and electrically connected with the leadframe fingers 14a by leads 28. The mold compound 16 encloses the die 12 and a portion of the leadframe 14, and is cured in shape. The heat slug 18 is made of a high thermal conductivity material, such as copper and aluminum. Furthermore, the heat slug 18 may also comprise an electro-deposited coating on a surface thereof. The heat slug 18 illustrated in FIG. 1 is an Exposed Drop-in Heat Slug (EDHS), which has one side exposed to the exterior for efficiently dissipating the heat generated by the die 12 to the surrounding.
As shown in FIG. 2, the conventional heat slug 18 comprises a main body 22 and a plurality of protrusions 24. The main body 22 is positioned closely adjacent to the die attach pad 14b of the leadframe 14 for receiving the heat generated by the die 12, which is then dissipated through surfaces of the heat slug 18. The main body 22 can be put in direct engagement with the die attach pad 14b for transmitting the heat to the heat slug 18 via the die attach pad 14b. Alternatively, the main body 22 can be bonded to the die attach pad 14b by the mold compound 16 or an adhesive for transmitting the heat to the heat slug 18 via the die attach pad 14b and the mold compound 16 or the adhesive. Moreover, each protrusion 24 has a slot 26 for effecting tight engagement between the heat slug 18 and the mold compound 16.
However, the QFP package structure 10, when put in a high-temperature, high-humidity environment, is often subject to alteration of the structure due to high temperature and high humidity. Particularly, a gap may occur in delamination between the main body 22 of the heat slug 18 and the die attach pad 14b of the leadframe 14. For example, an excessive gap between the heat slug 18 and the die attach pad 14b is observed when the conventional QFP structure 10 is subject to a test of moisture sensitivity level 3 (MSL3) at 260° C. The gap spoils the tight engagement between the heat slug 18 and the leadframe 14 and retards transfer of the heat generated by the die 12 to the heat slug 18, damaging the performance of heat dissipation of the QFP structure 10. Moreover, an oversize gap not only reduces the expected heat dissipation efficiency of the package structure but may also cause physical damage to the package structure.